1. Field of the Invention
The invention relates generally to the control of fuel injection in an internal combustion engine and more particularly to the precise control of fuel injection in an internal combustion engine by analysis of the exhaust gases.
2. Discussion of Background
It is known how to continuously determine the injection time of an engine using a prior art control device where the injection time is a function of the pressure in the intake manifold or of the air flow at the intake. The determination is made from a straight control line defined by its slope and its ordinate at the beginning in the injection time diagram as a function of the pressure. These values are computed, at most, only during the period of engine tune up. But it is known that they can vary randomly over time as a function of various parameters, for example, clogging of the air filter which reduces the air flow for the same pressure. Therefore, it is necessary for a precise control to readjust the parameters of this straight control line periodically.
For this purpose, there is a process, called the "American process," described particularly in the article "A Closed-Loop A/F Control Model for Internal Combustion Engines" by Douglas R. Hambourg and Michael A. Shulman, published in 1980 by the "Society of Automotive Engineers, Inc." This process consists in using a probe called a "lambda probe" for analysis of the exhaust gases. The probe gives a signal which varies when there is a lack of oxygen in the exhaust gases, showing a richness exceeding value 1 (corresponding to a stoichiometric mixture). When this happens, the parameters of the straight control line are corrected as follows: if the pressure P at the intake is less than a determined threshold, a correction is applied only on the ordinate at the beginning of the straight line, while if the value of the pressure is greater than this threshold, a correction is applied only on the slope of the straight line. This process therefore is approximate. Actually, if existing conditions are maintained, the recomputed straight line always ends up going through the existing operating point, but a local anomaly can falsify the computation of all other points. Further, this process functions only with a unity richness, while problems of fuel saving and pollution are increasingly leading to using richnesses less than unity.
There is also known a process called "superinjection," described in French patent application No. 83 17 538 in the name of the present applicant, and which consists, when the injection is controlled at a richness less than unity, in periodically making a progressive increase of the richness until triggering of the gas analysis probe is obtained, then in coming back to the initial richness while maintaining the value of the relative increase of the injection time which was thus necessary. This, compared with the theoretical increase resulting from the desired richness, gives the necessary correction. The above mentioned patent application indicates how it is possible to avoid jerks resulting from this momentary incursion in richness by controlling the ignition advance. However, these superinjections should be sufficiently spaced in time, with a period, for example, of 10 minutes.
It would, of course, be possible, using the prior art processes, to consider combining the American process, even with a richness less than 1, with the superinjection process. However, in this case, the lack of precision of the American process would be increased more by the considerable increase of the readjustment period due to the superinjection process.